1. Field
The present disclosure relates generally to power converters and, more specifically, to non-isolated Buck converters.
2. Description of Related Art
Conventional wall outlets generally deliver a high voltage alternating current (ac) power that needs to be transformed to direct current (dc) power before being used to power many electronic devices. Switched mode power converters are commonly used due to their high efficiency, small size, low weight, and safety protection features, to convert the high voltage ac power to a regulated dc power. For example, switch mode power converters are commonly used in drive circuitry in the lighting industry to provide dc power for various lamp types, such as compact florescent lamps (CFLs), Halogen lamps, or light emitting diode (LED) bulbs.
Output regulation in a switch mode power converter is usually provided by sensing a feedback signal from the output of the converter and controlling the power converter in a closed loop. In different control methods, the feedback or control signal may be used to modulate a duty cycle of a switching waveform (referred to as pulse width modulation (PWM)), to change a switching frequency (referred to as pulse frequency modulation (PFM)), or to disable some of the cycles of the switching waveform generated by the power converter controller (referred to as on-off control). Any of these control methods would result in control of the dc output voltage or current versus the load and line variations.
The feedback signal can be directly or indirectly extracted from a sense circuit coupled to the dc output of the converter. In a non-isolated converter, the input of the power converter may be electrically coupled to the output (e.g., dc current is able to flow between input and output terminals of the converter) and the feedback signal may be directly measured in reference to a reference level of the controller. However, in an isolated converter, the output reference level (output ground) may be galvanically isolated (e.g., dc current is prevented from flowing between input and output terminals of the converter) from the controller and switch reference level (primary ground). In these examples, the output can be sensed indirectly through the use of an opto-coupler or magnetically coupled winding on the transformer core.
In applications where the output of the power converter can be accessible by the consumer, isolated converters are typically used to provide galvanic isolation between the output load and the input power line. However, in applications where the output of the power converter is not accessible by the consumer during normal operation, simpler non-isolated converters having lower component counts, weights, and sizes may be utilized. For example, non-isolated converters may be used in various lamp types, such as CFLs, Halogen lamps, or LED bulbs.
In a converter with a regulated output current, such as an LED driver, the current at the output may be monitored/sensed/measured to provide a feedback signal to the controller in order to control switching and the transfer of energy to the converter output. A typical way to measure the output current is to include a sense resistor at the output of the power converter such that the output current flows through the sense resistor. The resultant voltage across the sense resistor is proportional to the output current. However, the voltage drop across the sense resistor is typically large and often referenced to a voltage level different than that of the power converter controller. Thus, additional circuitry, such as an opto-coupler or a bias winding, is often needed to level shift the voltage across the sense resistor in order to interface with the controller.